For instance, the rolling bearing unit is used to bear rotatably the wheel of the vehicle on the suspension system. Also, the running condition stabilizing system of the vehicle such as the antilock brake system (ABS), the traction control system (TCS), or the like is widely used to secure the running stability of the vehicle. According to the running condition stabilizing system such as ABS, TCS, or the like, the running condition of the vehicle can be stabilized in the braking or accelerating operation. However, in order to secure the stability under more severe conditions, the brake and the engine must be controlled by adopting a greater amount of information that have an influence on the running stability of the vehicle.
In other words, in the case of the running condition stabilizing system such as ABS, TCS, or the like in the related art, because the brake or the engine is controlled by sensing the slip between the tire and the road surface, i.e., the so-called feedback control is employed, control of the brake or the engine is delayed even for a minute. In other words, the so-called feedforward control can prevent neither the generation of the slip between the tire and the road surface nor the so-called one-sided activation of the brake, i.e., the braking force is extremely different between the right and left wheels, to attain improvement of the performance under the severe conditions. In addition, such control cannot prevent the situation that the running stability of the truck, or the like is degraded owing to on the bad loading condition.
In order to handle such problems, it may be considered that the load measuring unit, which measures one or both of the radial load and the axial load applied to the wheel, should be built in the rolling bearing unit, which bears the wheel on the suspension system, to execute the above feedforward control, or the like. As the wheel supporting rolling bearing unit with the load measuring unit that is available in such case, the units set forth in JP-A-2001-21577 (hereinafter referred as Patent Literature 1, JP-A-3-209016 (hereinafter referred as Patent Literature 2, JP-A-2004-3918 (hereinafter referred as Patent Literature 3, JP-B-62-3365 (hereinafter referred as Patent Literature 4 are known in the related art.
In Patent Literature 1 out of them, the rolling bearing unit with the load measuring unit that can measure the radial load is set forth. In the case of this first example of the related-art units, radial displacements of the stationary outer ring and the hub both being rotated on the inner diameter side of the outer ring are measured by the non-contact type displacement sensor, and thus the radial load applied between the outer ring and the hub is sensed. The sensed radial load is utilized to inform the driver of the bad loading condition as well as to control appropriately the ABS.
Also, in Patent Literature 2, the structure for measuring the axial load applied to the rolling bearing unit is set forth. In the case of the second example of the related-art units set forth in Patent Literature 2, the load sensor is attached to a plurality of locations of the inner surface of the stationary-side flange provided onto the outer peripheral surface of the outer ring, which surround the threaded holes into the bolts are screwed to couple the stationary-side flange to the knuckle, respectively. These load sensors are put between the outer side surface of the knuckle and the inner side surface of the stationary-side flange in a state that the outer ring is supported/fixed onto the knuckle. In the case of the load measuring unit built in the rolling bearing unit in the second example of the related-art units, the axial load applied between the wheel and the knuckle is measured by the load sensors.
Also, in Patent Literature 3, such a structure is set forth that the displacement sensors affixed to four locations of the outer ring in the circumferential direction and the sensed ring having the L-shaped cross section and fitted/fixed onto the hub are provided, and then the displacement of the hub with respect to the outer ring is sensed in the radial direction and the thrust direction at four locations to sense the direction of the load applied to the hub and its magnitude based on the sensed values at respective portions.
In addition, in Patent Literature 4, such a method is set forth that the strain gauge for sensing a dynamic distortion is provided to the outer ring equivalent member a part of rigidity of which is lowered, then the revolution speed of the rolling elements is calculated from the passing frequency of the rolling elements sensed by the strain gauge, and then the axial load applied to the rolling bearing is measured based on the revolution speed.
In the case of the first example of the related-art structure set forth in above Patent Literature 1, the load applied to the rolling bearing unit is measured by measuring the radial displacement of the outer ring and the hub by using the displacement sensor. In this event, because an amount of radial displacement is minute, the high-precision sensor must be employed as the displacement sensor to measure the load with good precision. Since the high-precision non-contact type sensor is expensive, an increase in cost as the overall rolling bearing unit with the load measuring unit is inevitable.
Also, in the case of the second example of the related-art structures set forth in Patent Literature 2, the load sensors must be provided as many as the bolts that support/fix the outer ring onto the knuckle. For this reason, not only the load sensor itself is expensive but also a considerable increase in cost as the overall rolling bearing unit with the load measuring unit is inevitable. Also, in the structure set forth in Patent Literature 3, a cost is further increased rather than the structure set forth in Patent Literature 1 since the sensor is provided to four locations of the outer ring in the circumferential direction. In addition, in the method set forth in Patent Literature 4, it is possible that it becomes difficult to ensure the durability of the outer ring equivalent member since a part of rigidity of the outer ring equivalent member must be lowered.
Also, in the structure or the method set forth in any of Patent Literatures 1 to 4, the dedicated mechanism is provided to measure the load applied to the rolling bearing unit. Therefore, increases in cost and weight are inevitable.
Further, in JP-A-2004-77159 (hereinafter referred as Patent Literature 5), as the technology in connection with the present invention, the structure for sensing the run-out of the inner ring that supports the encoder by using the encoder, on the sensed surface of which the N pole and the S pole are aligned alternately, is set forth. In this event, in above Patent Literature 5, the technology to measure the load applied to the rolling bearing unit by utilizing the encoder is not described even though the description of the suggestion regarding to such technology is considered together.